Synthesis of raspberry ketone

Abstract

The application discloses a synthesis method of raspberry ketone and relates to the field of organic synthesis. Specifically, the method comprises the following steps: taking 4-hydroxy benzalacetone and hydrogen as reactants, and performing a hydrogenation reaction under the catalysis of Raney nickel; wherein the temperature of the hydrogenation reaction is 80-120 DEG C, the pressure is 2-4 MPa, and the reaction time is 2-5 h. The synthesis method is green and environment-friendly, and has high yield.

Description

Technical Field

[0001] This invention relates to the field of organic synthesis, and more particularly to a method for synthesizing raspberry ketones. Background Technology

[0002] Raspberry ketone, found in natural raspberry juice, is a widely used flavoring agent with a delicate fruity aroma both domestically and internationally. It can be used to formulate edible and daily-use flavorings. Raspberry ketone and its acetylated compounds are sex attractants for fruit flies and can also be synthesized for use as pesticides. Furthermore, raspberry ketone and epicatechin gallate, as a novel nutritional supplement combination, are primarily used to treat or prevent obesity or obesity-related symptoms. Ractopamine, synthesized from raspberry ketone, can be used as a cardiotonic drug to treat congestive heart failure, obesity, and muscle atrophy. Given the wide range of applications of raspberry ketone, the development of efficient synthetic processes has attracted considerable attention from chemical engineers.

[0003] In recent years, the synthesis of raspberry ketones has been mainly divided into natural synthesis and chemical synthesis. At the same time, with the application of biosynthesis technology, researchers at home and abroad have synthesized raspberry ketones from the perspective of biosynthesis. At present, the main chemical synthesis routes of raspberry ketones are as follows: (1) p-hydroxybenzaldehyde and acetone are used as raw materials. In an alkaline system, such as NaOH, KOH and other alkaline liquid phase systems, Claisen-Schmidt condensation is carried out, and then catalytic hydrogenation is performed to synthesize raspberry ketones. This method can obtain a high yield of raspberry ketones, but the amount of alkaline waste liquid is large; (2) The route of synthesizing raspberry ketones from phenol and butanone alcohol (such as CN108530280A, CN106748694A, etc.) is used. The precursor raw material butanone alcohol is expensive, and the synthesis cost is high; (3) The route of catalytic synthesis of raspberry ketones from p-chlorophenol and 3-hydroxybutene is used. This method requires the use of precious metal catalysts, which makes the reaction cost high. Therefore, developing a green and efficient new method for synthesizing raspberry ketones has important practical application value. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a method for synthesizing raspberry ketone with high yield, low cost and low waste generation.

[0005] To address the technical problem of this invention, this invention provides a method for synthesizing raspberry ketone, which includes a hydrogenation reaction using 4-hydroxybenzylacetone and hydrogen as reactants under the catalysis of Raney nickel.

[0006] The hydrogenation reaction is carried out at a temperature of 80–120°C, a pressure of 2–4 MPa, and a reaction time of 2–5 h.

[0007] As an improvement to the above technical solution, the temperature of the hydrogenation reaction is 90-100℃, the pressure is 2.5-3.5MPa, and the reaction time is 2-3h.

[0008] As an improvement to the above technical solution, it also includes a step of using the MgO-Al2O3 complex as a catalyst to catalyze the condensation of p-hydroxybenzaldehyde and acetone to obtain 4-hydroxybenzylacetone.

[0009] The MgO-Al2O3 complex is obtained by calcining precursors prepared from aluminum and magnesium salts via a sol-gel method.

[0010] As an improvement to the above technical solution, the preparation method of the MgO-Al2O3 composite is as follows:

[0011] Magnesium nitrate and aluminum nitrate were dispersed in water to obtain a dispersion.

[0012] Adding an alkaline solution dropwise to the dispersion yields a precipitate, which is the precursor; wherein the alkaline solution is a NaOH solution or a KOH solution.

[0013] The precursor was calcined in air at 500–800°C for 1–5 h to obtain the MgO-Al2O3 complex.

[0014] As an improvement to the above technical solution, in the MgO-Al2O3 composite, the molar ratio of MgO to Al2O3 is 1:1 to 2:1.

[0015] As an improvement to the above technical solution, in the step of using MgO-Al2O3 complex as a catalyst to catalyze the condensation of p-hydroxybenzaldehyde and acetone to obtain 4-hydroxybenzylacetone, the reaction temperature is 250-350℃, and the molar ratio of p-hydroxybenzaldehyde to acetone is 1:1-1:1.5.

[0016] As an improvement to the above technical solution, in the step of using MgO-Al2O3 complex as a catalyst to catalyze the condensation of p-hydroxybenzaldehyde and acetone to obtain 4-hydroxybenzylacetone, the reaction temperature is 250-300℃, and the molar ratio of p-hydroxybenzaldehyde to acetone is 1:1.1-1:1.3.

[0017] As an improvement to the above technical solution, in the step of using MgO-Al2O3 composite as a catalyst to catalyze the condensation of p-hydroxybenzaldehyde and acetone to obtain 4-hydroxybenzylacetone, the condensation reaction of p-hydroxybenzaldehyde and acetone is carried out in a fixed-bed tubular reactor, the height of the catalyst bed is 50-80 mm, the length of the reaction tube is 100-200 mm, and the average particle size of the MgO-Al2O3 composite is 0.5-1 mm.

[0018] As an improvement to the above technical solution, in the step of using the MgO-Al2O3 complex as a catalyst to catalyze the condensation of p-hydroxybenzaldehyde and acetone to obtain 4-hydroxybenzylacetone, p-hydroxybenzaldehyde is dispersed in ethanol to obtain a first feed solution; propanol is dispersed in ethanol to obtain a second feed solution; the first feed solution is injected from the top of the reaction tube at a flow rate of 30-100 μL / min, and the second feed solution is injected into the vaporization chamber of the fixed-bed reactor at a flow rate of 30-100 μL / min.

[0019] The molar concentration of p-hydroxybenzaldehyde in the first raw material solution may be the same as or different from the molar concentration of propanol in the second raw material solution.

[0020] As an improvement to the above technical solution, the concentration of p-hydroxybenzaldehyde in the first raw material solution is 0.8-1.5 mol / L, and the concentration of acetone in the second raw material solution is 0.8-1.5 mol / L.

[0021] Implementing this invention has the following beneficial effects:

[0022] The present invention provides a method for synthesizing raspberry ketones using 4-hydroxybenzylacetone and hydrogen as reactants, and a hydrogenation reaction catalyzed by Raney nickel. This synthesis method is green and environmentally friendly, and yields high results. Furthermore, the present invention employs a composite metal oxide solid catalyst for the continuous catalytic synthesis of the raspberry ketone intermediate 4-hydroxybenzylacetone. This method is a continuous catalytic process with no waste alkaline discharge and high production efficiency. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to specific embodiments.

[0024] This invention provides a method for synthesizing raspberry ketone, which includes a hydrogenation reaction using 4-hydroxybenzylacetone and hydrogen as reactants under the catalysis of Raney nickel.

[0025] The hydrogenation reaction is carried out at a temperature of 80–120°C, with exemplary values ​​of 85°C, 90°C, 95°C, 100°C, 105°C, 110°C, 115°C, or 118°C, but not limited thereto. Preferably, the temperature is 90–100°C, and more preferably, it is 95–100°C.

[0026] The pressure for the hydrogenation reaction is 2–4 MPa, with exemplary values ​​being 2.3 MPa, 2.6 MPa, 2.9 MPa, 3.2 MPa, and 3.5 MPa. Preferably, it is 2.5–3.5 MPa, and more preferably, it is 2.8–3 MPa.

[0027] The hydrogenation reaction time is 2 to 5 hours, with exemplary times being 2.3 hours, 2.7 hours, 3 hours, 3.3 hours, 3.6 hours, 3.9 hours, 4.2 hours, 4.5 hours, or 4.8 hours, but not limited thereto. Preferably, it is 2 to 4 hours, more preferably 2 to 3 hours, and even more preferably 2.2 to 2.8 hours.

[0028] Based on the above reaction, the yield of raspberry ketone is ≥97%, and preferably can reach ≥98%.

[0029] Preferably, in some embodiments of the present invention, the method for synthesizing raspberry ketone further includes the step of preparing 4-hydroxybenzylacetone. Specifically, this includes: catalyzing the condensation of p-hydroxybenzaldehyde and acetone with a metal oxide solid base catalyst.

[0030] The metal oxide solid base catalyst is selected from one or more of MgO, Al2O3, MgO-Al2O3 complex, CaO, and CaO-Al2O3 complex, but is not limited thereto. Preferably, the metal oxide solid base catalyst is one or more of MgO, Al2O3, and MgO-Al2O3 complex. With these metal oxide solid base catalysts, the yield of 4-hydroxybenzylacetone can be controlled to be above 70%. More preferably, the metal oxide solid base catalyst is an MgO-Al2O3 complex, which can control the yield of 4-hydroxybenzylacetone to be above 90%.

[0031] Preferably, in some embodiments of the present invention, the method for synthesizing raspberry ketone includes the step of catalyzing the condensation of p-hydroxybenzaldehyde and acetone with an MgO-Al2O3 complex as a catalyst to obtain 4-hydroxybenzylacetone; that is, using the synthetic route shown in the following formula:

[0032]

[0033] The MgO-Al2O3 composite is obtained by calcining precursors prepared from aluminum and magnesium salts via a sol-gel method. The molar ratio of MgO to Al2O3 is 0.8:1 to 3:3, with exemplary ratios including 0.8:1, 0.9:1, 1:1, 1.2:1, 1.5:1, 1.8:1, 2:1, 2.4:1, 2.8:1, 0.8:1.5, 1.2:1.3, 1.5:2, 2:2.3, or 2.8:2.7, but not limited to these. A preferred ratio is 1:1 to 2:1, and a more preferred ratio is 1.25:1 to 2:1.

[0034] Specifically, in one embodiment, the preparation method of the MgO-Al2O3 complex is as follows:

[0035] (1) Disperse magnesium nitrate and aluminum nitrate in water to obtain a dispersion;

[0036] The molar ratio of magnesium nitrate to aluminum nitrate is (0.8–3):(2–6), such that the molar ratio of MgO to Al2O3 in the MgO-Al2O3 complex is 0.8:1–3:3. Exemplary molar ratios of magnesium nitrate to aluminum nitrate include 0.8:2, 0.9:2, 1:2, 1.2:2, 1.5:2, 1.8:2, 2:2, 2.4:2, 2.8:2, 0.8:3, 1.2:2.6, 1.5:4, 2:4.6, or 2.8:5.4, but are not limited thereto. A preferred ratio is (1–2):2, and a more preferred ratio is (1.25–2):2. By controlling the ratio of the two components, the yield of 4-hydroxybenzylacetone can be effectively increased. The concentration of magnesium nitrate in the dispersion is 0.2–1 mol / L, and the concentration of aluminum nitrate is 0.2–1 mol / L; preferably, the concentration of magnesium nitrate is 0.3–0.8 mol / L, and the concentration of aluminum nitrate is 0.3–0.8 mol / L.

[0037] (2) Add an alkaline solution dropwise to the dispersion, and the resulting precipitate is the precursor;

[0038] The alkaline solution is either NaOH or KOH, but is not limited to these. NaOH is preferred. The concentration of the alkaline solution is 0.5–1.5 mol / L, preferably 0.8–1.2 mol / L, and more preferably 0.8–1 mol / L.

[0039] (3) The precursor is calcined in air atmosphere to obtain MgO-Al2O3 complex.

[0040] The calcination temperature is 500–800°C, with exemplary values ​​of 540°C, 580°C, 620°C, 660°C, 700°C, 740°C, or 780°C, but not limited thereto. Preferably, it is 550–700°C, and more preferably, it is 550°C–650°C.

[0041] The calcination time is 1 to 5 hours, with exemplary values ​​being 1.3 hours, 1.6 hours, 1.9 hours, 2.2 hours, 2.6 hours, 3 hours, 3.5 hours, 3.8 hours, 4.1 hours, 4.5 hours, or 4.8 hours, but not limited thereto. Preferably, it is 1.5 to 3.5 hours, and more preferably, it is 1.5 to 2.5 hours.

[0042] Preferably, in some embodiments of the present invention, the product obtained by roasting is crushed to a particle size of 0.5 to 1 mm to facilitate subsequent loading, increase the reaction area, and improve catalytic efficiency.

[0043] Specifically, in the step of using the MgO-Al2O3 complex as a catalyst to catalyze the condensation of p-hydroxybenzaldehyde and acetone to obtain 4-hydroxybenzylacetone, the reaction temperature is 220-350°C, exemplary values ​​are 230°C, 250°C, 270°C, 290°C, 310°C, 330°C or 340°C, but not limited thereto, preferably 250°C-350°C, more preferably 250°C-300°C.

[0044] Specifically, in the step of using the MgO-Al2O3 complex as a catalyst to catalyze the condensation of p-hydroxybenzaldehyde and acetone to obtain 4-hydroxybenzylacetone, the molar ratio of p-hydroxybenzaldehyde to acetone is 1:1 to 1:1.5, exemplary ratios are 1:1.05, 1:1.15, 1:1.2, 1:1.25, 1:1.3 or 1:1.45, but not limited thereto, preferably 1:1 to 1:1.3, more preferably 1:1.1 to 1:1.2.

[0045] Specifically, in some embodiments of the present invention, in the step of catalyzing the condensation of p-hydroxybenzaldehyde and acetone to obtain 4-hydroxybenzylacetone using an MgO-Al2O3 complex as a catalyst, the condensation reaction of p-hydroxybenzaldehyde and acetone is carried out in a fixed-bed tubular reactor. The height of the catalyst bed is 50-80 mm, the length of the reaction tube is 100-200 mm, and the average particle size of the MgO-Al2O3 complex is 0.5-1 mm. Using a fixed-bed tubular reactor promotes effective contact between the catalyst and the reactants, increasing the yield. The 4-hydroxybenzylacetone synthesis method of the present invention has low temperature sensitivity, allowing the use of a fixed-bed tubular reactor.

[0046] Specifically, in the step of catalyzing the condensation of p-hydroxybenzaldehyde and acetone to obtain 4-hydroxybenzylacetone using the MgO-Al2O3 complex as a catalyst, p-hydroxybenzaldehyde is dispersed in ethanol to obtain a first feed solution; propanol is dispersed in ethanol to obtain a second feed solution; the first feed solution is injected from the top of the reaction tube at a flow rate of 30–100 μL / min, and the second feed solution is injected into the vaporization chamber of the fixed-bed reactor at a flow rate of 30–100 μL / min. Based on the above injection method, the yield of 4-hydroxybenzylacetone can be further improved.

[0047] The concentration of p-hydroxybenzaldehyde in the first feed solution is 0.8–1.5 mol / L, and the concentration of acetone in the second feed solution is 0.8–1.5 mol / L.

[0048] Based on the preparation steps of 4-hydroxybenzylacetone described above in this invention, the yield of 4-hydroxybenzylacetone can be increased to over 97%. Simultaneously, the overall yield of raspberry ketone after the two-step reaction is ≥95%.

[0049] The present invention will be further described below with reference to specific embodiments:

[0050] It should be noted that the fixed-bed reactors used in the following embodiments are all adiabatic single-stage fixed-bed reactors, which are FD-2000 models of fixed-bed reactors produced by Hunan Huasi Instrument Co., Ltd., with a height of 100mm, an inner diameter of 8mm, and a catalyst bed height of 60mm.

[0051] Example Group 1

[0052] This set of examples provides the preparation of MgO-Al2O3 composite catalysts, specifically including:

[0053] Aluminum nitrate and magnesium nitrate were dissolved in deionized water at a predetermined molar ratio (total molar concentration of aluminum nitrate and magnesium nitrate was 1 mol / L). A 1 mol / L NaOH solution was slowly added dropwise under stirring. A composite hydroxide was prepared in the alkaline system via a sol-gel method. The composite hydroxide was filtered, dried, and calcined at 600℃ in air for 1.5 h to obtain a MgO-Al2O3 alkaline solid composite catalyst. The catalyst was crushed and sieved to obtain particles of 0.5–1 mm for later use. Based on the ratio of aluminum nitrate to magnesium nitrate, MgO-Al2O3 composites with molar ratios of MgO to Al2O3 of 1:0, 1:2, 1:1, 2:1, and 0:1 were prepared. These were designated as MgO-Al2O3(1:0), MgO-Al2O3(1:2), MgO-Al2O3(1:1), MgO-Al2O3(2:1), and MgO-Al2O3(0:1), respectively.

[0054] Example 2

[0055] This embodiment provides a method for synthesizing 4-hydroxybenzylacetone, specifically including:

[0056] The MgO-Al2O3 (1:0) composite catalyst prepared in Example 1 was packed into a fixed-bed tubular reactor and heated to 280–290 °C. An ethanol solution of p-hydroxybenzaldehyde (22.2 g p-hydroxybenzaldehyde dissolved in 100 mL of ethanol) was injected from the top of the reaction tube at a flow rate of 50 μL / min (via a syringe pump), and an ethanol solution of acetone (5.8 g acetone dissolved in 100 mL of ethanol) was introduced from the vaporization chamber of the fixed-bed reactor at a flow rate of 60 μL / min (via a syringe pump). The catalytic reaction was carried out to synthesize 4-hydroxybenzylacetone, with a gas-phase yield of 85%.

[0057] Example 3

[0058] This embodiment provides a method for synthesizing 4-hydroxybenzylacetone, specifically including:

[0059] The MgO-Al2O3 (1:2) composite catalyst prepared in Example 1 was packed into a fixed-bed tubular reactor and heated to 280–290 °C. An ethanol solution of p-hydroxybenzaldehyde (22.2 g p-hydroxybenzaldehyde dissolved in 100 mL of ethanol) was injected from the top of the reaction tube at a flow rate of 50 μL / min (via a syringe pump), and an ethanol solution of acetone (5.8 g acetone dissolved in 100 mL of ethanol) was introduced from the vaporization chamber of the fixed-bed reactor at a flow rate of 60 μL / min (via a syringe pump). The catalytic reaction was carried out to synthesize 4-hydroxybenzylacetone, with a gas-phase yield of 91%.

[0060] Example 4

[0061] This embodiment provides a method for synthesizing 4-hydroxybenzylacetone, specifically including:

[0062] The MgO-Al2O3 (1:1) composite catalyst prepared in Example 1 was packed into a fixed-bed tubular reactor and heated to 280–290 °C. An ethanol solution of p-hydroxybenzaldehyde (22.2 g p-hydroxybenzaldehyde dissolved in 100 mL of ethanol) was injected from the top of the reaction tube at a flow rate of 50 μL / min (via a syringe pump), and an ethanol solution of acetone (5.8 g acetone dissolved in 100 mL of ethanol) was introduced from the vaporization chamber of the fixed-bed reactor at a flow rate of 60 μL / min (via a syringe pump). The catalytic reaction was carried out to synthesize 4-hydroxybenzylacetone, with a gas-phase yield of 93%.

[0063] Example 5

[0064] This embodiment provides a method for synthesizing 4-hydroxybenzylacetone, specifically including:

[0065] The MgO-Al2O3 (2:1) catalyst prepared in Example 1 was packed into a fixed-bed tubular reactor and heated to 280–290 °C. An ethanol solution of p-hydroxybenzaldehyde (22.2 g p-hydroxybenzaldehyde dissolved in 100 mL of ethanol) was injected from the top of the reaction tube at a flow rate of 50 μL / min (via a syringe pump), and an ethanol solution of acetone (5.8 g acetone dissolved in 100 mL of ethanol) was introduced from the vaporization chamber of the fixed-bed reactor at a flow rate of 60 μL / min (via a syringe pump). The catalytic reaction was carried out to synthesize 4-hydroxybenzylacetone, with a gas-phase yield of 97%.

[0066] Example 6

[0067] This embodiment provides a method for synthesizing 4-hydroxybenzylacetone, specifically including:

[0068] The MgO-Al2O3(0:1) composite catalyst prepared in Example 1 was packed into a fixed-bed tubular reactor and heated to 280–290 °C. An ethanol solution of p-hydroxybenzaldehyde (22.2 g p-hydroxybenzaldehyde dissolved in 100 mL of ethanol) was injected from the top of the reaction tube at a flow rate of 50 μL / min (via a syringe pump), and an ethanol solution of acetone (5.8 g acetone dissolved in 100 mL of ethanol) was introduced from the vaporization chamber of the fixed-bed reactor at a flow rate of 60 μL / min (via a syringe pump) to carry out the catalytic reaction, synthesizing 4-hydroxybenzylacetone. The gas-phase yield of 4-hydroxybenzylacetone was 75%.

[0069] Example 7

[0070] This embodiment provides a method for synthesizing 4-hydroxybenzylacetone, specifically including:

[0071] The MgO-Al2O3 (2:1) composite catalyst prepared in Example 1 was packed into a fixed-bed tubular reactor and heated to 280–290 °C. An ethanol solution of p-hydroxybenzaldehyde (22.2 g p-hydroxybenzaldehyde dissolved in 100 mL of ethanol) was injected from the top of the reaction tube at a flow rate of 50 μL / min (via a syringe pump), and an ethanol solution of acetone (5.8 g acetone dissolved in 100 mL of ethanol) was introduced from the vaporization chamber of the fixed-bed reactor at a flow rate of 55 μL / min (via a syringe pump) to carry out the catalytic reaction, synthesizing 4-hydroxybenzylacetone with a gas-phase yield of 95%.

[0072] Example 8

[0073] This embodiment provides a method for synthesizing 4-hydroxybenzylacetone, specifically including:

[0074] The MgO-Al2O3 (2:1) composite catalyst prepared in Example 1 was packed into a fixed-bed tubular reactor and heated to 280–290 °C. An ethanol solution of p-hydroxybenzaldehyde (22.2 g p-hydroxybenzaldehyde dissolved in 100 mL of ethanol) was injected from the top of the reaction tube at a flow rate of 50 μL / min (via a syringe pump), and an ethanol solution of acetone (5.8 g acetone dissolved in 100 mL of ethanol) was introduced from the vaporization chamber of the fixed-bed reactor at a flow rate of 65 μL / min (via a syringe pump). The catalytic reaction was carried out to synthesize 4-hydroxybenzylacetone, with a gas-phase yield of 96.8%.

[0075] Example 9

[0076] This embodiment provides a method for synthesizing raspberry ketone, specifically including:

[0077] The condensed liquid material from the outlet of the fixed-bed tubular reactor in Example 5 was collected and added to a high-pressure reactor. At the same time, Raney nickel catalyst was added and hydrogen gas at 3 MPa was introduced. Catalytic hydrogenation was carried out at 100°C for 2.5 h to obtain raspberry ketone with a gas phase yield of 98%.

[0078] The above description is a preferred embodiment of the invention. It should be noted that, for those skilled in the art, several improvements and modifications can be made without departing from the principle of the invention, and these improvements and modifications are also considered to be within the scope of protection of the invention.

Claims

1. A method for synthesizing raspberry ketone, characterized in that, include: Using an MgO-Al2O3 complex as a catalyst, 4-hydroxybenzylacetone was prepared by condensation of p-hydroxybenzaldehyde and acetone; wherein, in the MgO-Al2O3 complex, the molar ratio of MgO to Al2O3 was 1:1 to 2:1; the reaction temperature was 220 to 350 °C, and the molar ratio of p-hydroxybenzaldehyde to acetone was 1:1 to 1:1.5; The hydrogenation reaction was carried out using 4-hydroxybenzylacetone and hydrogen as reactants under the catalysis of Raney nickel; the hydrogenation reaction temperature was 80~120℃, the pressure was 2~4MPa, and the reaction time was 2~5h. The preparation method of the MgO-Al2O3 composite is as follows: Magnesium nitrate and aluminum nitrate were dispersed in water to obtain a dispersion. Adding an alkaline solution dropwise to the dispersion yields a precipitate, which is the precursor; wherein the alkaline solution is a NaOH solution or a KOH solution. The precursor was calcined in air at 500-800°C for 1-5 hours to obtain the MgO-Al2O3 complex.

2. The method for synthesizing raspberry ketone as described in claim 1, characterized in that, The hydrogenation reaction is carried out at a temperature of 90-100℃, a pressure of 2.5-3.5MPa, and a reaction time of 2-3h.

3. The method for synthesizing raspberry ketone as described in claim 1, characterized in that, The preparation method of the MgO-Al2O3 complex is as follows: Magnesium nitrate and aluminum nitrate were dispersed in water to obtain a dispersion. Adding an alkaline solution dropwise to the dispersion yields a precipitate, which is the precursor; wherein the alkaline solution is a NaOH solution. The precursor was calcined in air at 550-650°C for 1.5-2.5 h to obtain the MgO-Al2O3 complex.

4. The method for synthesizing raspberry ketone as described in claim 1, characterized in that, In the MgO-Al2O3 composite, the molar ratio of MgO to Al2O3 is 1.25:1 to 2:

1.

5. The method for synthesizing raspberry ketone as described in claim 1, characterized in that, In the step of using the MgO-Al2O3 complex as a catalyst to catalyze the condensation of p-hydroxybenzaldehyde and acetone to obtain 4-hydroxybenzylacetone, the reaction temperature is 250~350℃, and the molar ratio of p-hydroxybenzaldehyde to acetone is 1:1.1~1:1.

3.

6. The method for synthesizing raspberry ketone as described in claim 5, characterized in that, In the step of using the MgO-Al2O3 complex as a catalyst to catalyze the condensation of p-hydroxybenzaldehyde and acetone to obtain 4-hydroxybenzylacetone, the reaction temperature is 250~300℃, and the molar ratio of p-hydroxybenzaldehyde to acetone is 1:1.1~1:1.

2.

7. The method for synthesizing raspberry ketone according to any one of claims 2 to 6, characterized in that, In the step of using MgO-Al2O3 composite as a catalyst to catalyze the condensation of p-hydroxybenzaldehyde and acetone to obtain 4-hydroxybenzylacetone, the condensation reaction of p-hydroxybenzaldehyde and acetone is carried out in a fixed-bed tubular reactor. The height of the catalyst bed is 50-80 mm, the length of the reaction tube is 100-200 mm, and the average particle size of the MgO-Al2O3 composite is 0.5-1 mm.

8. The method for synthesizing raspberry ketone as described in claim 7, characterized in that, In the step of catalyzing the condensation of p-hydroxybenzaldehyde and acetone to obtain 4-hydroxybenzylacetone using MgO-Al2O3 composite as catalyst, p-hydroxybenzaldehyde is dispersed in ethanol to obtain a first feed solution; propanol is dispersed in ethanol to obtain a second feed solution; the first feed solution is injected from the top of the reaction tube at a flow rate of 30~100 μL / min, and the second feed solution is injected into the vaporization chamber of the fixed-bed reactor at a flow rate of 30~100 μL / min. The molar concentration of p-hydroxybenzaldehyde in the first raw material solution may be the same as or different from the molar concentration of propanol in the second raw material solution.

9. The method for synthesizing raspberry ketone as described in claim 8, characterized in that, The concentration of p-hydroxybenzaldehyde in the first feed solution is 0.8~1.5 mol / L, and the concentration of acetone in the second feed solution is 0.8~1.5 mol / L.

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